A long-standing problem associated with the use of heat pumps in most climatic regions of the world is that frequently the outdoor coil will, during the heating mode of operating, have ice accumulate thereon. As the ice thickness increases, the overall efficiency of the heat pump system decreases significantly, and a substantial amount of energy may be wasted. Accordingly, many arrangements have been proposed heretofore for detecting the ice and for taking corrective action for removing the ice from the outdoor coil. Examples of prior art systems include the following U.S. Pat. Nos.: 3,170,304; 3,170,305; 3,400,553 and 4,209,994.
It has been recognized that, for a given set of criteria, there is an optimum point (of ice built up) at which to command a defrost mode of operation of the heat pump system. If defrost is commanded too soon or too late, energy will be wasted, i.e., total system efficiency will suffer.
The present invention is a defrost control system which addresses the problem which is sometimes encountered wherein a demand type defrost control system will, for various reasons, demand too frequently a defrost mode of operation. Demand type defrost control systems are old in the art. A first type compares the outdoor coil inlet air temperature with the outlet air temperature thereof and whenever the differential of these temperatures exceeds a certain preselected value, then defrost is commanded. Another prior art demand system is to compare the outdoor air temperature with the outdoor coil temperature and upon a preselected differential between such temperatures the defrost mode of operation is commanded. Also, another demand system is to compare the air pressure associated with the inlet and outlet of the outdoor coil, i.e., the air passing through the coil and, whenever the differential pressure between such points exceeds a certain preselected value, the defrost mode of operation is commanded. As indicated, there can be problems associated with such demand type systems which will cause unrequired defrost operations which in turn results in a plummeting of the overall system efficiency. With a differential temperature control system, for example, an evaporator fan failure will cause a large differential of temperature and thereby command numerous defrosts each hour of time rather than only several per day. A similar situation occurs for both the differential temperature and the differential pressure methods if the evaporator clogs up because of leaves, dirt, a snowdrift, or the inability of the heat pump to defrost because of other factors.